Multiple Myeloma (MM) is characterized by increased angiogenesis and bone destruction, which both result in enhanced tumor growth. Blocking either tumor associated angiogenesis or bone resorption by osteoclasts (OCL) significantly impacts MM growth in preclinical models. Further, the recent Medical Research Council Myeloma IX trial, which compared zoledronic acid and clodronate, both inhibitors of OCL activity, in combination with chemotherapy in patients with newly diagnosed MM, showed that zoledronic acid treatment significantly increased survival by 5.5 months compared to the weaker bisphosphonate clodronate, and that this survival benefit was independent of skeletal-related events. Importantly, 30% of the patients in this trial did not have lytic bone disease at the start of the trial and still benefited from zoledronic acid therapy. These results suggest that blocking OCL activity may have additional inhibitory effects on tumor growth beyond effects on bone resorption. Consistent with this possibility, we recently reported that OCL are angiogenic cells both in vivo and in vitro, suggesting that OCL may contribute to the enhanced angiogenesis in MM. In support of this hypothesis, we have demonstrated that Annexin 2 (AXII) is secreted by OCL, is a growth factor for MM cells and can stimulate proliferation of human endothelial cells. These results suggest that OCL have a multiplicity of effects in myeloma that result in enhanced tumor growth and bone destruction. However, the role of OCL-derived AXII in MM is unclear. It is our hypothesis that the increased OCL activity present in MM patients increases tumor growth, both through secretion of growth factors and angiogenic factors such as AXII by OCL and release of MM growth factors from the bone matrix. Further, release of AXII from bone marrow stromal cells and OCL increases production of cytokines by MM cells that induce osteoclastogenesis, osteoblast suppression, and angiogenesis. Therefore, to test this hypothesis, we will assess the role of microenvironment-derived AXII in myeloma bone disease (MMBD) by pursuing the following specific aims: 1) Determine if modulating AXII produced in the MM microenvironment by bone marrow stromal cells and OCL impacts the capacity of MM cells to induce osteoclastogenesis, osteoblast suppression, and angiogenesis. 2) Determine if OCL-derived AXII acts as an angiogenic factor. 3) Determine the role of marrow microenvironment-derived AXII on angiogenesis, tumor growth and bone destruction in MM using our recently developed immunocompetent in vivo model of MMBD. As part of this aim, we will determine the contribution of OCL-derived AXII to these processes to test the hypothesis that OCL make multiple contributions to tumor growth, bone destruction and the increased angiogenesis characteristic of MM in vivo. Thus, In this proposal, we will determine the roles of both bone marrow stromal cell-derived and in particular OCL-derived AXII activity in MMBD and their respective roles in regulating MM cell functions including osteoblast suppression, enhanced osteoclastogenesis, and production of pro-angiogenic factors (AIM 1); determine the capacity of OCL-derived AXII's capacity to act directly as an angiogenic factor on endothelial cells (AIM 2); and determine the role of microenvironment-derived AXII (total knockout) and specifically of OCL-derived AXII (conditional knockout) in tumor growth, bone destruction and angiogenesis in vivo (AIM 3).